Reception of signals on a loop antenna



y 1945- c. G. SONTHEIMER ET AL 2,375,593

RECEPTION OF SIGNALS ON A LOOP ANTENNA Filed April 1, 1945 Isnnentors 68M Gttorneg Patented May 8, 1945 RECEPTION OF SIGNALS ON A LOOP ANTENNACarl G. Sontheimer, Haddonfield, and Robert S. Doak, Collingswood, N.J., assignors to Radio Corporation of America, a corporation of Dela-Application April 1, 1943, Serial No. 481,406

3 Claims. (01. 250-33) This invention relates to 100p antennas, and moreparticularly to a method of and means for providing increased signalpickup for a given loop inductance. The figure of merit of a loopantenna for use with radio receivers, direction finders, and the like,is the ratio of effective height to the square root of the loopinductance. The effective height of a loop represents the factor which,when multiplied by the field strength in microvolts per meter, gives ameasure of the E. M. F. generated in the loop. The effective height,h=21r7 A/ meters, where n=number of turns, and A=area of each turn insquare meters.

The inductance, L, of a loop antenna is approximately proportional toWA. Thus for a loop of given area, both the effective height and thesquare root of the inductance are substantially proportional to thenumber of turns, and

the figure of merit, h/ L, is substantially independent of the number ofturns on the loop. The power extracted from a radiation field by a loopdepends basically on the radiation resistance of the loop; but thiseffect is ordinarily masked by the inductance of the loop.

The principal object of the present invention is to provide an improvedloop construction and method whereby the advantages of increasedeffective height may beobtained without corresponding increase in theinductance.

A further object is to provide loop antennas having a greater ratio ofeffective height to the square root of inductance than prior loopantennas. g E

Referring to the drawing, Fig. 1 illustrates the construction of oneembodiment of the invention, and Fig. 2 shows a modified construction.

A closed core I, of magnetically permeable material such as comminutediron oxide, powdered iron mixed with a suitable binder material, or thelike, is provided with a winding comprising coils 3 and 5 disposed onthe legs 1 and 9 respectively of the core I. The coils are connected inseries in reverse polarities to each other between terminal II and I3,so that the mutual inductance between the two legs of the winding issuch as to diminish the net series inductance.

The arrows Hs on the drawing represent the magnetic flux of a radiationfield linking the coils 3 and 5. The voltages 8s induced in the twocoils are additive, i. e. the output voltage of the loop issubstantially twice that of either coil.

The series inductance of the two coils is:

=L1+L22K\/Ll L2 where L1 and L2 are the self inductances of the coils 3and 5 respectively, and K is the coefiicient of coupling between them.If the permeability of the core I were infinite, L would be zero, andthe total voltage would still be the sum of the voltages induced in thetwo coils. The expression 2K\/L1 L2 represents the mutual inductancewhich is opposed to either of the self inductances. The effectiveness ofloops constructed in accordance with the invention is thus largelydependent upon the permeability of the core material.

Referring to Fig. 2, a core iii of toroidal shape is employed. Thewindings are disposed on this core in a manner similar to thearrangement of the windings 3 and 5 on the core of Fig. l. The operationof this structure is identical with that of the loop of Fig. 1. Itshould be apparent without further illustration that either of thewindings may be wound in a reverse direction, provided the connectionsthereto are interchanged.

Thus the invention has been described as an improved loop antennastructure, comprising spaced coils coupled together by means of a closedmagnetic core to provide mutual inductance which is opposed to the selfinductances of the spaced coils. This arrangement provides animprovement in the ratio of effective height to inductance over priorartstructures.

We claim as our invention:

1. A directive antenna comprising a closed core of magneticallypermeable material, and windings disposed on spaced axes on said coreand connected in series with each other in such relationship that themutual inductance between said windings is opposed to the selfinductances thereof.

2. A directive antenna comprising a closed core of magneticallypermeable material, and windings disposed on spaced parallel axes onsaid core and connected in series with each other in such relationshipthat the mutual inductance between said windings is opposed to the selfinductances thereof.

3. A directive antenna comprising a substantially toroidal core ofmagnetically permeable material, and windings disposed on spaced arcs onsaid core and connected in series with each other in such relationshipthat the mutual inductance between said windings is opposed to the selfinductances thereof.

CARL G. SON'I'HEIMER. ROBERT S. DOAK.

